GaN, AlN, and InN, which are nitride semiconductors, or materials made of mixed crystals thereof, have a wide band gap, and are used as high output electronic devices or short-wavelength light emitting devices. For example, GaN that is a nitride semiconductor has a band gap of 3.4 eV, which is wider than the band gap 1.1 eV of Si and the band gap 1.4 eV of GaAs.
As such a high output device, there is a Field effect transistor (FET), more particularly, a High Electron Mobility Transistor (HEMT) (see, for example, Patent Document 1). A HEMT using such a nitride semiconductor is used for high output/high efficiency amplifiers and high power switching devices. Specifically, in a HEMT using AlGaN as the electron supply layer and GaN as the electron transit layer, piezoelectric polarization occurs in AlGaN due to strains caused by the difference in lattice constants between AlGaN and GaN, and high-density two-dimensional electron gas (2DEG) is generated. Thus, operations at high voltage are possible, and the HEMT may be used for high-efficiency switching elements and high breakdown voltage electric devices in electric vehicles, etc.
The 2DEG generated as described above is usually in a region immediately below the gate, even in a state where voltage is not applied to the gate electrode, etc., and therefore the fabricated device becomes normally-on. However, it is generally desired that a power switching element, etc., is normally-off; that is to say, when the gate voltage is 0 V, it is desired that there is no current flowing between the drain-source. Therefore, various structures and methods have been studied to make the device become normally-off (for example, Patent Document 2). Furthermore, in order to suppress a leak current in a transistor such as HEMT, there is disclosed a device having an insulating gate structure in which an insulating film is formed under the gate electrode (for example, Patent Document 3).
Patent document 1: Japanese Laid-Open Patent Publication No. 2002-359256
Patent document 2: Japanese Laid-Open Patent Publication No. 2011-14789
Patent document 3: Japanese Laid-Open Patent Publication No. 2010-199481
Incidentally, in a HEMT using a nitride semiconductor having a structure in which an insulating film is formed under the gate electrode, the gate threshold voltage may become unstable, which is unfavorable as operation failures may be caused. Furthermore, in such a HEMT, the gate leak current is preferably low.